1. Field of the Invention
The present invention relates generally to electrical systems and devices, and more particularly to apparatus and methods for detecting resistance changes in connectors to a remote mounted sensor.
2. Description of Related Art
In a typical water heater control system, a remote mounted sensor is used to detect water temperature inside a water heater tank. In some systems, the remote mounted sensor is a Positive Temperature Coefficient (PTC) or Negative Temperature Coefficient (NTC) thermistor. Generally, a thermistor is a temperature sensitive passive semiconductor that exhibits a large change in electrical resistance when subjected to a relatively small change in temperature. The resistance of a PTC thermistor goes up as the temperature increases; whereas the resistance of an NTC thermistor goes down as the temperature increases. Such a sensor can be electrically connected to a microcontroller that monitors the resistance of the thermistor, converts the resistance to a corresponding water temperature value in accordance with the positive or negative relationship of resistance to temperature, and adjusts the water heater operation to maintain the water temperature within a desired range.
In some systems, the remote sensor is electrically connected to the microcontroller by way of a quick connection means. Over time, such connection means can corrode, thereby increasing in resistance. The increased resistance of the connector translates to an increase in the overall resistance detected by the microcontroller, ultimately corrupting the accuracy of the temperature detection function. For example, in a sensor comprising a PTC thermistor, an increase in the resistance of the connector would cause the microcontroller to detect a temperature that is higher than that actually sensed by the thermistor. In contrast, an increase in the resistance of the connector in an NTC thermistor circuit would cause the microcontroller to detect a temperature that is lower than that actually sensed by the thermistor. As a result, the accurate temperature detection and control of the water temperature are compromised by the increased resistance of the connector. In particular, in the case of an NTC thermistor circuit, such an increased resistance would tend to cause the microcontroller to unnecessarily increase the water temperature. For safety reasons, it is generally preferable to avoid unintended increases in temperature.
One approach for checking a temperature sensing circuit is addressed in U.S. Pat. No. 5,768,429 to Peterson. The object of the invention in Peterson is to provide a temperature sensing circuit having a safe failure mode and employing resistive temperature sensors suitable for high limit heating plant temperature detection. In Peterson, a temperature sensing circuit is described as including two switches and parallel NTC and PTC thermistors, where the NTC thermistor is also coupled in series with a diode. Failure of one of the resistive temperature sensors may be detected by monitoring charging and discharging time constants associated with the resistive temperature sensors. A failure is evidenced by a dramatic change in the ratios of the time constants. During discharge, if the diode is reverse biased, current flows through one thermistor, and if the diode is forward biased, current flows through both thermistors. Peterson does not, however, disclose or suggest determining or detecting a change in, a connector resistance of a connector coupling a remote mounted thermistor to a microcontroller.
Therefore, need exists for a means of detecting changes in resistance of the connectors to a remote mounted sensor. Furthermore, need exists for a means of determining the accurate resistance of the a remote mounted sensor, notwithstanding changes in connector resistance.